Backward extrusion method and product

ABSTRACT

A method of forming a closed-end vessel by backward extrusion comprises providing an extrusion billet having a front surface with an axial recess and a body of an extrudable material positioned in the recess. Backward extrusion results in the formation of a closed-ended vessel, for example a pressurized gas cylinder, composed of the material of the extrusion billet with a weld bonded inner surface lining of the extrudable material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns composite closed-end vessels, and theirproduction by backward extrusion.

2. Discussion of Prior Art

The technique of backward extrusion involves the use of a generallycylindrical container with parallel side walls, and a ram to enter thecontainer dimensioned to leave a gap between itself and the side wallsequal to the desired thickness of the extrudate. An extrusion billet ispositioned in the container. The ram is driven into a forward face ofthe billet and effects extrusion of the desired hollow body in abackwards direction. The forward motion of the ram stops at a distancefrom the bottom of the container equal to the desired thickness of thebase of the extruded hollow body. Extrusion speed, the speed at whichthe extrudate exits from the container, is not critical but is typicallyin the range 50-500 cm/min. Lubrication can substantially reduce theextrusion pressure required.

SUMMARY OF THE INVENTION

In one aspect, this invention concerns a development of this technique.The invention provides a backward extrusion method for forming aclosed-ended vessel which comprises providing, in a container forbackward extrusion, a billet of a first extrudable metal, said billethaving an axis and a forward face, and driving a ram along the axis intothe forward face of the billet,

wherein the forward face of the billet is made with an axial recess anda body of a second extrudable material is provided in the recess,

whereby there is formed a closed-ended vessel composed of the firstextrudable material with an adherent inner surface lining of the secondextrudable material.

In another aspect, the invention provides a pressurised gas containerformed by backward extrusion, which container is composed of analuminium alloy and carries a weld bonded inner surface lining of anextrudable material.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is directed to the accompanying drawings in which:

FIGS. 1 and 2 are sectional side elevations of backward extrusionequipment according to the invention at different stages in the backwardextrusion process.

FIGS. 3 and 4 are sectional side elevations of extrusion billets, eachhaving a forward face with an axial recess therein.

FIGS. 5 and 6 are plan and side elevations of a body of a secondextrudable material to be provided in the recess.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, backward extrusion equipment comprises a container10 having cylindrical side walls to contain an extrusion billet 12, anda ram 14. The extrusion billet has a front face 16 provided with ashallow axial recess defined by a rim 18 surrounding the recess. A body20 of a second extrudable material is provided in the recess. The ram ismounted for reciprocation in a direction 22 along the axis of theextrusion billet and the container.

FIG. 2 shows the position after the ram has been driven into the forwardface of the extrusion billet. There has been formed by backwardextrusion a closed-ended vessel 24 having cylindrical side walls. Thevessel is composed of the first extrudable metal 26, derived from thebillet 12, with a weld bonded inner surface lining of the secondextrudable material 28 derived from the body 20.

If a cylindrical extrusion billet of first material had been placed inthe container, and a disc of the second material placed on top of it,then the backward extrusion operation would have resulted in aclosed-ended vessel in which the second material was concentrated at theforward end of the cylindrical wall, with little or none forming aninterior lining at the closed backward end. To avoid this, the extrusionbillet 12 is formed with an axial recess in its forward face, with thebody of the second material being positioned in that recess. Preferablyno part of the body of the second material stands proud of the extrusionbillet. Preferably the extrusion billet includes an annular part whichsurrounds and extends forward of the recess in which the body of thesecond material is provided. Preferably the diameter of the axial recessin the forward face of the extrusion billet is substantially equal tothe diameter of the ram. These features can be used to ensure that thefirst and second materials are co-extruded from the start, and inparticular that the second material is not extruded prior to the firstone.

Preferably the body of the second extrudable material is shrink-fittedin a correspondingly shaped recess in the top surface of the extrusionbillet. Thus a cold body of second material may be inserted into acorresponding recess in a hot extrusion billet, which then cools andcontracts round the body. This shrink-fitting arrangement hasadvantages: a) the interfacial region between the billet and the body ismaintained free from lubricant ingress, and b) the shrink-fittingprocess establishes a local residual stress pattern that favours theinitiation of co-extrusion at the start of the back-extrusion process.

The process of backward extrusion results in the formation of aclosed-ended vessel composed of the first extrudable material with aweld bonded inner surface lining of the second extrudable material. Theweld bonding is a metallurgical bond that results from the backwardextrusion process; for example, deposition of metal by electrolytic orother means would result in a lining but not one weld bonded to thesubstrate. The lining may be present on the entire inner surface of theclosed-ended vessel. Alternatively, the lining may be present only atthe closed end and on the cylindrical side wall adjacent the closed end.Control over this may be achieved by controlling the shape and depth ofthe recess into which the body of the second material is inserted priorto extrusion.

The extrusion billet is of a first extrudable material which ispreferably a metal for example an aluminium alloy. Conventionalextrudable Al alloys, such as those from the 2000, 6000 and 7000 seriesof the Aluminum Association Inc Register, are suitable.

Provided in a recess on that extrusion billet is a body, e.g. a sheet,disc, slab or block of a second extrudable material, preferably onewhich is more extrudable than the first. This material may be selectedfrom a wide range in order to impart desired surface properties to theextrudate. For example it may be an extrudable metal of differentcomposition to the extrusion billet e.g. Al or Ni or a different Alalloy when the extrusion billet is of an Al alloy; or an organicpolymer, or a metal matrix composite. If this material would causedamage on contact with the extrusion equipment, it may be sheathed orotherwise protected so as to prevent such contact.

The backward extrusion process may be performed with the extrusionbillet preferably cold or warm, or even hot. The extrusion conditionsare not material to this invention, and conventional conditions may beused.

For simplicity, the invention has hitherto been described on the basisthat only two different materials are co-extruded. But of course bodiesof many different materials may be provided overlying one another in theextrusion container, so as to obtain a composite extrudate in which thewalls comprise layers of the many different materials.

This invention thus provides a route to generate multi-layer laminatedextruded structures offering unique combinations of properties, forexample:

Low or high weight to stiffness and/or volume ratios,

Outstanding toughness and fatigue crack growth resistance,

Controllability of fracture modes,

Internal surface layers with specific properties,

All by a low-cost production route.

The invention allows use of materials in back-extruded products thatare:

a) Incompatible with direct contact with the extrusion punch-nose butcan offer beneficial properties. Thus for example, metal matrixcomposites (MMC) would promote excessive punch-nose wear duringextrusion but would provide high specific stiffness in products.Problems with extrudate materials being incompatible with the extrusioncontainer or sleeve can be overcome by placing the extrusion billetsections within a suitable thin walled tube.

b) Too chemically reactive for long-term exposure to the envisagedservice environment but offer desirable properties in the final product,e.g. specific strength, stiffness and toughness. (Special steps may beneeded to overcome problems associated with exposed laminate material atthe open end of the extruded shell).

c) Outside the chemical composition ranges of current alloyspecifications. This should permit the utilisation of recycled scrapalloy.

d) Beneficial to a structure but deficient in at least one propertypre-requisite for a particular application.

Design and fabrication of safe and weight efficient high pressure gascontainment systems impose very demanding material propertyrequirements, almost inevitably resulting in at least one propertyhaving to be compromised to allow achievement of the required propertybalance. The above invention offers a method to minimise these materialselection restrictions thereby allowing the fabrication of novel systemstailored to provide specific properties, for example:

a) Internal surfaces can be engineered to be inert or re-active in aparticular combination of gas, liquid and solid phases.

Some manufacturers currently market high pressure gas containers formedby backward extrusion of an excess-silicon alloy designated 6351. Theywould like to move to a balanced alloy 6061. But some customers areresistant to this move, because they believe that a minor copperaddition in 6061 may have a detrimental influence on the long term gasstability provided by aluminium high-pressure gas cylinders. Thisconcern (real or imaginary) can be addressed by means of this inventionby providing an internal cladding of an Al alloy of differentcomposition overlying the whole of the internal wall and end surfaces ofthe container.

b) Outer and/or sandwich layers with desirable properties (e.g. highstiffness, wear resistance, strength, etc. from a MMC) can be providedby materials that would have caused unacceptable tool wear duringextrusion. This is achieved by using a billet top-sheet to preventpunch-nose contact with the abrasive material during backward extrusion.

c) Chemically reactive materials offering a particularly desirableproperty can be sandwiched between layers providing adequate resistanceto chemical attach, e.g. lithium rich Al--Li based alloys, magnesiumbased alloys or aluminium scrap alloys containing unusually high levelsof iron, silicon and/or a combination of other alloying elements.

d) A suitable designed laminated structure can significantly improveboth the fracture and fatigue performance of a high pressure gascylinders as it is possible to include layer(s) with specific propertiesand to introduce boundary interfaces ensuring that cracks initiating inone layer will be blunted at laminate boundary with significantreduction of the stress intensity promoting crack propagation. In thecase of the fatigue of gas cylinders it is envisaged that the use ofappropriate laminated structures will markedly improve cylinderperformance, because crack initiation and growth resistances aregenerally controlled by the performance of material at the internalknuckle-radius of the cylinder base to wall transition region which willbe readily modified using multi-layer extrusion billets during backwardextrusion.

EXAMPLE 1

An experimental run was performed with the object of extruding twodifferent aluminium alloys at the same time. The extrusion billet was ofa 7XXX alloy and on top of that was provided a disc of 1100 aluminium.

Two slugs were extruded detailed as follows:

1. The first extrusion goal was to yield a 7XXX shell with a wall of 104mm mean with an 1100 inner liner of 0.25 mm thickness.

Results: There was some deformation at the opening of the cup followedby what appears to be a continuous lining of 1100 aluminium throughoutthe inside of the 7XXX shell.

2. The second extrusion goal was to yield a 7XXX shell with a wall of101 mm mean with an 1100 inner liner of 0.50 mm thickness.

Results; The end of the cup shattered upon impact of the ram, but thecup completed extrusion. It appears there is a lining throughout thelength of the 7XXX shell.

In both cases the liner thickness tapers from approximately 0.10 mm atthe open end to less than 0.025 mm or 0.05 mm at the base end.

EXAMPLE 2

An experimental run was performed with the object of extruding twodifferent aluminium alloys at the same time. The main extrusion billetwas a 7000 series alloy (Al; 6% Zn; 2% Mg; 2% Cu; 0.2% Cr). The insertmaterial was commercially pure aluminium sheet (1100). The extrusionbillet is shown in FIG. 3. This is a cylindrical billet 20 cm diameterand 25 cm long. In the forward face (top in the drawing) a torisphericalrecess is machined of shape corresponding to the shape of the ram. Thediameter of the recess is 18.04 cm and the depth of the recess is 5.375cm.

The insert is shown in FIGS. 5 and 6. This is a disc 18.02 cm diameterand either 0.625 or 1.250 cm thick.

The 7000 extrusion billet surfaces (other than the recess) werelubricated using a stearate based paste, and a disc of the insertmaterial was placed in the machined recess and its outer surfacelubricated.

During the initial stages of extrusion, while the 1100 flat sheet wasdeforming to the shape of the 7xxx series billet's machined profile, itwas found that an air-pocket was trapped between the two alloys and aloud noise resulted when extrusion process eventually forced the air toescape to the atmosphere. It was also observed that the 1100 alloyextruded to some degree prior to the two alloys co-extruding. Thiseffect was more pronounced for the thicker 1100 inserts and thisaccounts for why the 1100 thickness on the internal surfaces of theextruded shells were independent of insert thickness.

The approximately 100 cm long cylindrical shells (wall thickness 10.7mm) formed by backward extrusion, resembled those formed when monolithicbillets are extruded, save that in this case the 7xxx series alloyshells were lined with a thin layer of commercially pure aluminium. The1100 alloy layer thickness was tapered, being thickest (0.1 mm) at thestart of the extrusion, i.e. the open-end of the shell and the thinnest(0.025-0.05 mm) at the closed-end, which was formed at the end of theextrusion. The internal surface finish of the cylindrical shells wasexcellent, resembling that of a dull mirror. The surface condition wassuperior to that typically produced when 7xxx or 6xxx series alloys areback-extruded under similar conditions. Metallographic examination ofthe shell walls confirmed that a metallurgical bond had been createdbetween the 7xxx and 1100 alloys during co-extrusion for all regionsother than towards the open-end of the extrusion, which formed duringthe early stages of the extrusion. This is consistent with lubricant andtrapped air being present in the interfacial region between the 1100alloy plate insert and the 7xxx series billet at the start of theextrusion process.

EXAMPLE 3

The extrusion billets used in this further trial were as shown in FIG.4. Each 6061 billet was pre-machined with a axial 5 cm deep recesscomprising a 18.44 cm diameter flat-base hole with a slightly smallerdiameter flat-base hole in its base. The depth of the smaller hole was0.125 cm greater than the thickness of the 1100 disc employed in theextrusion trial as an insert.

The 1100 alloy discs were inserted in two ways, one involving the discsbeing machined to size and simply placed into position while the otherinvolved shrink-fitting slightly oversized diameter discs into the 6061billets by inserting discs into pre-heated (150° C.) 6061 ingotrecesses. Prior to back-extrusion the billets were lubricated using astearate based product.

                  TABLE 1                                                         ______________________________________                                        1100 alloy disc and machined recess sizes for extrusion trials                          Disc Diameter                                                                            Machined Recess                                                                            Disk Thickness                              Disc Location                                                                           (cm)       Diameter (cm)                                                                              (cm)                                        ______________________________________                                        As-Machined                                                                             17.95      17.96        0.625                                                 18.02      18.04        1.25                                        Shrink-Fit                                                                              17.95      17.92        0.625                                                 18.02      18.00        1.25                                        ______________________________________                                    

Although all the variants evaluated yielded approximately 100 cm longco-extruded 6061 extruded shells with a thin layer of 1100 alloy on theinternal wall surfaces, the shrink-fitted discs consistently gave asuperior result.

For the shrink-fit case:

a) Co-extrusion of the two alloys initiated immediately at the start ofbackward extrusion with the 1100 alloy layer being flush with the 6061and

b) the 1100 layer was continuous along the entire length of the shelland had a polished "mirror" finish.

Results for the as-machined fitted discs were less reproducible. The1100 alloy layer had a dull appearance and there was often evidence ofpoor adhesion between the 6061 and the 1100 layers with blistersoccurring due to air being trapped between the two alloys. In addition,unlike for the shrink-fit case, the 1100 material always started toextrude prior to co-extrusion conditions being established. In someinstances, particularly when 1.25 cm thick 1100 inserts were used, highpercentages of the 1100 was extruded prematurely, thereby beingunavailable for co-extrusion.

The main reasons why the shrink-fitted inserts give a superior resultare:

a) the interfacial region between the 6061 billet and the 1100 alloyinsert are maintained free from lubricant ingress and

b) the shrink-fitting process establishes a local residual stresspattern that favours the initiation of co-extrusion at the start of theback-extrusion process.

As expected the 1100 alloy layers produced during co-extrusion weretapered, being thickest at the open of the shell and thinnest at theclosed-end. Continuous 1100 alloy layers were found on the closed-end ofall the shells produced, independent of the 1100 disk thickness orinsertion method involved. In the case of shells formed from the billetswith as-machined fitted insert disks, although these 1100 alloy layerswere extremely thin, they were readily recognisable in the shell baseregions because of the local surface blistering characteristics.

The open ends of co-extruded shells formed from two 6061 billets withshrunk-fitted 0.625 cm thick 1100 alloy discs were hot swaged to formthe crown region of high pressure gas cylinders. This process involvedcropping the open-end of the shell by 10-12 cm, annealing the remainingfirst 15-20 cm of shells open-end at 450° C. for a few seconds prior tohot swaging the end in a heading die at the same temperature to form acylinder crown. Subsequent metallographic examination of these cylindersrevealed that the hot swaging process had not degraded the coherencebetween the 6061 and 1100 alloys and that 6061 high pressure aluminiumalloy gas cylinders with a continuous internal surface layer ofcommercially pure aluminium alloy 1100 may be fabricated by the methodoutlined in this example.

We claim:
 1. A backward extrusion method for forming a closed-ended vessel for use as a high pressure gas container which comprises providing, in a container for backward extrusion, a billet of a first extrudable metal, said billet having an axis and a forward face, lubricating the billet, and driving a ram along the axis into the forward face of the billet,wherein the forward face of the billet is made with an axial recess and a body of a second extrudable metal is provided in the recess so as to exclude lubricant from the interface between said first extrudable metal and said second extrudable metal, whereby there is formed a closed-ended vessel composed of the first extrudable metal with an adherent inner surface lining of the second extrudable metal.
 2. A method as claimed in claim 1, wherein said step of providing said second extrudable metal in said recess comprises the step of shrink-fitting said second extrudable metal in said recess in the top surface of the billet.
 3. A method as claimed in claim 1, wherein the billet of the first extrudable metal includes an annular part which surrounds and extends forward of the recess in which the body of the second extrudable metal is provided.
 4. A method as claimed in claim 1, wherein the ram has a diameter substantially equal to the diameter of the axial recess in the forward face of the billet.
 5. A method as claimed in claim 1, wherein the first extrudable metal is an aluminium alloy.
 6. A method as claimed in claim 1, wherein the body of a second extrudable metal is a metal disc.
 7. A method as claimed in claim 1, wherein the adherent inner surface lining of the second extrudable metal is present on the entire inner surface of the closed-ended vessel.
 8. A backward extrusion method for forming a closed-ended vessel, said method comprising the steps of:providing a billet of a first extrudable metal in a container for backward extrusion, said billet having an axis and a forward face, said billet having an axial recess in said forward face; locating a body of a second extrudable metal in said recess so as to seal any space between said first and second extrudable metals from any subsequently applied extrusion lubricant; lubricating said billet with an extrusion lubricant; and backward extruding said billet and said second extrudable metal so as to provide said closed end vessel.
 9. A method as claimed in claim 8, wherein said step of locating said second extrudable metal into said recess comprises the step of shrink-fitting said second extrudable metal into said recess.
 10. A method as claimed in claim 8, wherein said second extrudable metal does not extend into a plane including the forward face of the first extrudable metal. 